A UK Government position paper has said that temporary "backstop" arrangements to prevent a hard border between Northern Ireland and the Republic due to Brexit should not continue beyond December 2021.

Here are some key questions answered.

What is the backstop?

Downing Street is planning to prevent a hard frontier at the UK's only land border with an EU state after Brexit if no preferred trade agreement is reached on withdrawal.

Ensuring frictionless passage of goods and services through the border is one of the most vexed issues facing negotiators and the backstop is the Government's option of last resort if current talks fail.

Maintaining customs harmony would make border checks less likely or unnecessary but keeping common food or other regulatory standards covered by the EU single market for goods and services could pose other challenges.

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What options are on the table?

Brexiteers like Boris Johnson oppose a "customs partnership" with the EU, whereby the UK would collect tariffs set by the EU customs union on goods entering the country on behalf of the bloc.

The other possibility is maximum facilitation and, rather than scrapping customs checks, would use technology to minimise the need for them.

Brexit Secretary David Davis has talked of trusted trader arrangements, which could allow companies to pay duties in bulk every few months rather than every time their goods cross a border, and automated number plate recognition.